The invention relates to injection molding machines, such as the type that produce heat cured or thermo-setting materials such as but not limited to silicone rubber. Thermoset injection units are typically built upon a screw based injection system originally designed for thermoplastic polymers. A typical thermoplastic injection system first takes material in a pellet form from a hopper and feeds it into a heated barrel where a screw rotates to convey, compress, and mix the material before plunging to force the material into the mold cavity where the material cools to become solid in order to make a part. Upon the introduction of thermoset materials, the thermoplastic injection system was adapted to inject thermoset material using many of the same elements. A typical thermoset injection first takes material in a liquid form under pressure and feeds it into a barrel where a screw rotates to convey the material before plunging to force the material into the mold cavity where the material is heated to cure and become solid in order to make a part.
Some innovations in thermoset injection systems have been focused on keeping the material at a low enough temperature before it is injected so it will not cure before entering the mold cavity. One innovation is to build a mold in more than two plates with insulation material between the plate where the thermoset material first enters the mold and the heated mold cavity plate. One disadvantage of this approach is that is can increase the cost and complexity of the mold design. A further innovation is to pump liquid through cavities in the plates to be cooled allowing a large amount of heat to be removed. One disadvantage of liquid cooling is that if the pumping system fails, the thermoset material can cure before entering the mold cavity causing the entire machine to stop working.
Other innovations are focused on metering the amount of thermoset material which enters the mold. One such innovation is a check valve with a spring to keep it shut. Placed at the nose of the screw, a typical check valve had no spring and would open from material pressure difference upon the screw being drawn back, then close from material pressure difference when the screw would plunge forward. By adding a spring, the check valve would only open from material pressure difference upon the screw being drawn back, then close from once the spring force overcame the material pressure difference before the screw would plunge forward. This allows for more accurate metering of the material exiting the barrel, as the check valve will open and close more consistently. One disadvantage of using a check valve as a shutoff is that it can still be challenging to accurately meter because the shutoff is located at the tip of the screw, and the material may have to go through several passages before entering the mold cavity. A further innovation is to put independently controlled needle shutoffs placed directly in the mold base. This allows for more accurate metering because having the shutoff located physically closer to the mold cavity reduces the volume of material being metered. One disadvantage of this innovation is that can increase the cost and complexity of the mold base.
Other innovations are focused on reducing the amount of voids in the final product due to trapped air or additional processes to remove extra material called flash. A typical mold has small passages to allow air inside the mold cavity to escape as material enters the mold cavity. Molds designed this way may produce parts with trapped air if material enters the mold cavity faster than the air can escape. It is undesirable to have voids in the part as it may adversely affect the physical properties or the aesthetics. Molds designed this way may also produce parts with additional material, called flash, if the material flows into the passages meant to allow the air to escape. It is undesirable to have flash on the part as it may require additional processing to remove. One innovation is to use a vacuum pump to remove air from the mold cavity before the material enters. The mold is then designed without air passages and a vacuum hole outside of the mold cavity area on one side of the mold and a gasket surrounding the mold cavity and the vacuum hole on one side of the mold. The mold clamping mechanism then controls the mold parting distance to the point where the gasket seals between the two halves, but the mold is not completely closed so not to plug the vacuum hole. The mold parting distance is then kept at this position until the vacuum pump has drawn out the air, then the mold is closed fully which plugs the vacuum hole and closes the parting distance. The material is then injected into the mold cavity. The advantages of this mold design are that because there is no air in the mold, the part will not have trapped air, and because there are no air passages, the part will have no flash. Some disadvantages of this mold design are that it requires an additional gasket in the mold and precise control over the mold parting distance from the clamping mechanism.
In summary, many of the innovations have focused on correcting the shortcomings of a thermoset injection system which was adapted from a thermoplastic injection system. Having an injection system designed from the ground up to respect the unique processing requirements of thermoset materials would result in a significant reduction in time, cost, and space and a significant increase in speed, accuracy, and reliability.
Quick change tooling has received much commercial success in many manufacturing processes including injection molding. The ‘Master Unit Die’ or ‘MUD’ has become the industry standard, one such version is shown in U.S. Pat. No. 5,562,935. A popular U-frame style Master Unit Die with companion mold tool inserts are shown in the drawings as an example of one type of mold that can be used with the injection molding machine. The success of the MUD system is due to the benefits of both faster tooling change over and lower tooling cost since a common mold base is reused. There are several patents and technology in the area of quick change tooling, however, this does not address the problem of production downtime related to material change over, especially to a different color.
In plastic injection molding plastic pellets are pre-heated in a hopper then a screw in a heated barrel turns and strokes the material into the mold. When switching materials or colors the entire injection system needs to be dismantled to fully clean the screw, barrel, nozzle and all components that come in contact with the material. Purging techniques exist but are not sufficient to completely clear out the injection system. An injection system tear down and cleaning can typically take a full day leaving the injection molding machine unavailable for production. These same plastic molding machines are used to mold thermoset materials like liquid silicone rubber (“LSR”) with some minor modifications. However, a complete tear down for cleaning is still required for both material change over and at the end of the production run since the material could cure to the point of locking the components together if left in the machine for days. Cleaning uncured thermoset material from injection components is more involved and time consuming as solvents are often needed to remove the sticky material that is comparable to tree sap.
Prior art clamping systems for plastic injection mold machines are typically servo hydraulic, servo electric systems or a combination. The systems often include toggle mechanisms for mechanical advantage when applying a clamp force. These systems are costly, large and heavy especially in presses that are 10 tons and higher. Hydraulic systems are not very energy efficient since the hydraulic pump is running all of the time. Electric servo systems have improved efficiency but often require 3 phase electric power to operate and are typically more expensive to purchase that hydraulic systems. Toggle systems add expense, weight and take up space. Generating clamp force with different mold stack up heights is further complicated since the mechanical advantage of the toggle changes with platen position. Somewhat less costly hydro-pneumatic cylinders and similar air over oil intensifier systems can provide a high clamping force without the need for toggle mechanisms but do not have any means to provide programmable motion control for opening and closing the platens. Programmable motion control of the platen is needed for many molding applications.
A need exists to have a simple compact, low cost platen clamping system that can generate clamping force and have a means for programmable motion control for opening and closing the platens including the ejection process.